Look for rich color and plumpness with a slight softness along the seam of the fruit.  Most varieties have a orange yellow color between red areas, but some varieties have a greenish back round instead of yellow orange.  Bright looking fruits which are firm to moderately hard will probably ripen normally in 2 to 3 days at room temperature.  avoid hard, dull fruits, or those with cracked or punctured skin, or other signs of decay.  Russeting or staining of the skin may affect the appearance but not detract from the internal quality of the nectarine.